While the invention is subject to a wide range of applications, it is particularly suited for the assembly of electronic packages. In particular, the invention relates to the assembly of metal pin grid array packages by vacuum lamination.
Microelectronic devices are typically manufactured from a semiconductor material such as silicon, germanium or gallium/arsenide. The semiconductor material is fashioned into a die, a generally rectangular structure having circuitry formed on one surface. Along the periphery of that surface are input/output pads to facilitate electrical interconnection to external components.
The device is brittle, requiring protection from moisture and mechanical damage. The protection is provided by an electronic package. The package further contains electrical conductors to transport electrical pulses between the device and external circuitry.
One package design which minimizes space requirements and provides a high density of electrical interconnections is a pin grid array (PGA) package. One pin grid array package has a multilayer ceramic substrate with conductive circuitry disposed between the layers. The circuitry terminates at conductive pads to which terminal pins are brazed. The pins are configured in a regular array. U.S. Pat. No. 4,821,151 to Pryor et al. discloses a ceramic pin grid array package.
A molded plastic pin grid array package is disclosed in U.S. Pat. No. 4,688,152 to Chia. A printed wire board having plated through holes forms the package base. An integrated circuit device is bonded to one face of the printed circuit board. Lead wires electrically interconnect the device to circuit traces patterned on the board. The circuit traces terminate at conductive rings containing terminal pins which pass through the board. The surface containing the integrated circuit, wire bonds and circuit traces is then encapsulated in a molding resin.
Another molded plastic pin grid array package is disclosed in U.S. Pat. No. 4,965,227 to Chang et al. A circuit tape having terminal pins is soldered to circuit traces supported by a dielectric layer. The terminal pins and circuit tape are then partially encapsulated in a polymer resin.
Ceramic pin grid array packages have excellent reliability. However, the ceramic packages are brittle, expensive and poor conductors of heat. One advantage of pin grid array packages is a high density of electrical connections. The high density allows the housing of complex integrated circuits and hybrid devices which generate heat during operation. If the heat is not removed, the device operating life is shortened. It has been estimated that for every 10.degree. C. increase in operating temperature, the effective operating life is decreased by fifty percent.
Plastic pin grid array packages are not as brittle and are cheaper to manufacture than ceramic packages. The thermal performance of a plastic PGA may be enhanced by molding a metallic heat spreader into the body of the package. The large surface area of exposed plastic makes plastic PGA packages susceptible to moisture permeation.
One package which provides moisture resistance and reliability at a reduced cost with high thermal performance is a metal PGA. A metal PGA is disclosed in U.S. Pat. No. 5,103,292 which is incorporated herein in its entirety. The metal PGA has a metallic base with an ordered array of holes. The package cover is usually made from the same metal as the base. A circuit tape is disposed between the base and cover. A plurality of terminal pins are electrically interconnected to the circuit tape. The terminal pins extend outwardly through the ordered array of holes in the package base. A dielectric bonds the cover component to both the circuit tape and to the base component, as well as the base component to the circuit. The dielectric extends into the ordered array of holes electrically isolating the pins from the base.
The package base is preferably a metal with high thermal conductivity such as a copper or aluminum alloy. The electronic device is bonded to the base so the entire base is a heat spreader. Thermal conduction is improved over that obtained by molding a heat spreader into the base of a plastic PGA.
The dielectric is preferably a polymer adhesive such as a thermosetting epoxy. During assembly, the adhesive flows into the ordered array of holes formed in the base electrically isolating the terminal pins from the base. To optimize the performance of the metal PGA, adhesive reaction products must be eliminated or removed. During bonding, the polymerization of the epoxy resin generates reaction by-products. Residual contaminants such as chlorides and bromides are present from the manufacture of the adhesive. In the presence of water vapor, the by-products and residual contaminants form corrosive acids such as hydrochloric acid are formed. The corrosive acids may dissolve device metallization or corrode the wire bonds.
The terminal pins are electrically isolated from the metallic base by the adhesive. The electrical conductivity of the holes is reduced by depositing an anodization layer or other electrically resistive barrier on the walls, but primary insulation is provided by the adhesive. It is therefore desirable to maximize the flow of adhesive into the holes.
Vacuum lamination has been used to manufacture flexible and rigid printed wiring boards. The process has not been applied to the assembly of a PGA. With the incorporation of novel fixturing, vacuum lamination may be applied to the assembly of metal pin grid array packages.
Accordingly, it is an object of the invention to provide a fixture for the vacuum lamination of a pin grid array package. It is a further object of the invention to provide a method for the assembly of pin grid array packages which utilizes vacuum lamination. It is a feature of the invention that the fixture comprises a cavity plate, cover plate and grid hole plate. Yet another feature of the invention is that a vacuum is applied to the base of the package through the grid hole plate.
It is a benefit of the invention that the polymer resin is drawn into the ordered array of holes formed in the package base for enhanced electrical resistance. Yet another advantage of the invention is that the concentration of volatile reaction by-products is reduced. Still another benefit of the invention is that the number of voids in the polymer is reduced. A further benefit of the invention is that fixturing provides exact positioning of the package components. A stop in the cover plate assures uniform thickness. Another benefit of the invention is that vacuum lamination may be applied to adhesively sealed dual-in-line (DIP) packages.
In accordance with the invention, there is provided both a fixture for the assembly of pin grid array packages and a method of manufacture. The fixture is formed from a grid plate having a plurality of holes extending from a first side to a second side. A cavity plate having at least one aperture to receive a pin grid array assembly is supported by the first side of the grid plate. The fixture includes both a means to heat the assembly and a means to draw a vacuum on the grid plate.
The method of assembly includes the step of providing a package assembly. The package assembly contains a base component having a first array of holes, a cover component and a circuit disposed between the base and cover components. The circuit has an array of terminal pins electrically interconnected. The terminal pins pass through the first array of holes. A dielectric sealing means is disposed between the base component and the circuit. A second dielectric sealing means is disposed between the cover component and the circuit. A vacuum is applied to the base component. The package assembly is then heated to a temperature sufficient to cause the first and second dielectric sealing means to flow and to bond the base component to both the circuit and to bond the cover component to both the circuit and the base component.
The above stated objects, features and advantages will become more clear from the specification and drawings which follow.